如何在JLabel数组中添加

Question

I don't know what to do. Please help me

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;

    public class Sample{
    static JLabel label[];
    static int count = 0;

      public static void main (String [] args){
      JFrame frame = new JFrame("Sample");
      JTextField tf = new JTextField(10);
      JButton bt = new JButton("Okay");
      frame.setSize(800,600);
      frame.setLayout(new FlowLayout());
      frame.setVisible(true);
      frame.add(tf);
      frame.add(bt);


      bt.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent ae){
          int sum = count++;
          String getTF = tf.getText();
          label[count] = new JLabel(getTF);
        }
    });
      }
}

It says NullPointerException. How do I add getTF to label[] ? I am confused and I can't find the right answer someone might have something for me.

Answer 1

You have not initialized your array:

static JLabel label[];   // "label" reference is null

You need to initialize it with an actual array object, eg like

static JLabel label[] = new JLabel[10];

However, this will then have a fixed array size (10 in this case). You will not be able to add more than 10 elements to that array. You should use an ArrayList instead which is a dynamically growing array:

static List<JLabel> label = new ArrayList<>();

Then add your new label like

label.add(new JLabel(getTF));

Besides that, you should use static fields only if absolutely necessary - better create an instance of your application class and make the fields non-static.

You should also remove the wild card imports (like java.util.* ) and only import those classes and interfaces which you require (like java.util.List and java.util.ArrayList ). Otherwise you might get name clashes (like between java.awt.List and java.util.List )